The present invention relates to a multiphase meter system for measurements of composition and/or salinity of a fluid flow in a pipe.
More specifically the present invention relates to measurement of water, oil, gas and/or salinity fractions in a fluid flow in the exploration of oil and gas reservoirs.
A number of different commercial flow meters are available on the market for the measurement of the content of fluid flows from oil wells. Some meters are based on the use of radioactive radiation, some are capacitive, and some are based on the use of microwaves.
Microwave sensors are attractive because they are not limited by the health risks associated with radioactive radiation based meters and their fairly low accuracy or the undesirable influence of contamination on the capacitive sensors.
An example of a method for measuring properties of flowing fluids and a metering device and a sensor used for performing this method has been described in International Patent Application PCT/NO01/00200, for which a US-patent has been granted (U.S. Pat. No. 6,826,964 B2). The sensor uses the microwave resonance principle for the measurement of oil-continuous fluids (water drops and gas bubbles in oil, i.e. the oil is a continuous phase) and the measurement of conductivity for water-continuous fluids (oil drops and gas bubbles in water, i.e. the water is the continuous phase, and is intended for installation at a production zone inside an oil well. Another example of resonance measurements in multiphase flows is discussed in Norwegian patent 308922 (corresponding to WO99/63331).
Another example of a method for measuring flowing fluids with a far higher gas content, i.e. wet gas (a wet gas flow is a multiphase flow with a high gas volume fraction, usually called the gas void fraction (GVF), typically >99%) or high-gas multiphase flow, has been described in U.S. Pat. No. 6,915,707. This is also based on the microwave resonance principle.
The microwave resonance principle is based on measuring the permittivity/dielectric constant of the flow and is discussed with reference to water volume fractions (WVF) in WO 2008/085065.
Other systems are described in U.S. Pat. No. 5,101,163 and WO2007/018434, as well as U.S. Pat. No. 5,341,100, where the composition and water salinity is measured by measuring the difference in the signal received by two antennas being positioned at different distances from the transmitter antenna, called the transmission method. This method is usually preferred when the loss in the fluid flow is high, e.g. in a water continuous flow with relatively high water salinity.
A problem inherent in the known art is that it is difficult to provide sufficiently accurate measurements within the complete range of compositions and salinities within the same fluid volume. As is discussed above the resonant frequency and Q-factor is suitable for a certain range of fractions, but less suitable when the loss in the flow gets sufficiently high and the accuracy is reduced. This problem is solved as described in the claims.